So users of Gmail, Hotmail and other DMARC-enabled providers will not only fail to receive messages sent to the mailing list by Yahoo users, but will flood the list with bounce messages, risking to be bounced off the list themselves, Levine said.
The email expert recommended that mailing list operators suspend the list posting rights of yahoo.com users and ask them to re-subscribe to their lists with accounts from different email providers.
Yahoo did not immediately respond to an inquiry seeking clarification on whether its new DMARC policy is permanent or temporary.
A test of Yahoo's DMARC records Tuesday done with a tool on dmarcian.com revealed that the "p=reject" setting was still in place for the yahoo.com domain. By comparison, gmail.com had a policy record of "p=none," meaning it doesn't tell other email servers how to handle messages from gmail.com addresses that fail DMARC checks.
Laura Tessmer Atkins, co-founder of email anti-spam consultancy firm Word to the Wise based in Palo Alto, California, also confirmed and documented the issue in a blog post Monday. She believes that Yahoo began advertising a "reject" policy because of a recent attack against Yahoo users that involved attackers compromising yahoo.com email accounts and sending unauthorized emails to their contacts.
"The attackers have modified their attacks and are now sending mail from Yahoo users to their contacts through other servers," Atkins said. "By publishing a p=reject record, Yahoo is telling other systems to not accept mail from Yahoo users if it doesn't come through Yahoo controlled servers. This includes the mail from the attackers, but also mail from regular Yahoo users that use another SMTP server, including bulk mail sent through ESPs [email service providers], and individual mail sent to mailing lists."
DMARC.org, the industry group that oversees the development and adoption of the DMARC standard, did not immediately respond to a request for comment about the Yahoo situation. However, the frequently asked questions section of the group's website acknowledges the interoperability problems mailing lists can have with DMARC and offers some recommendations.
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